1. Field of the Invention
The invention relates to an ignition circuit for extinguishing tubes in electronic flash equipment for photography, having an automatic dosage detecting system for determining the quantity of light, wherein an extinguishing tube is employed in order to discharge a flash capacitor, or for extinguishing a series thyristor.
2. Description of the Prior Art
Electronic flash equipment for photography, having automatic light dosage or systems for controlling luminescence time, respectively, has been known in the trade for many years. The simplest method to control the luminous time of a flash tube is to parallel-connect an additional, dimmed flash tube -- a so-called extinguishing tube -- to the flash tube. This extinguishing tube is ignited as soon as the light reflected by the object being flashed has attained a pre-determined value. At this moment the extinguishing tube forms a low-ohmic parallel circuit path to the flash tube. The flash tube is extinguished, and the residual charge still present in the flash capacitoris destroyed in the extinguishing tube (See U.S. Pat. No. 3,857,064).
The fact that in the above-described circuit the residual charge still stored in the flash capacitor is unnecessarily destroyed has resulted in an additional circuit in which a controllable semi-conductor element, a thyristor preferably, is arranged in series to the flash tube. The thyristor is turned on together with the flash tube at the beginning of the flash process, and is turned off with the aid of an extinguishing circuit when the luminous time desired is attained. In this circuit modification in which the current through the flash tube is cut off means of the thyristor connected in series to the flash tube, which for this reason is also called a series thyristor, the residual charge still present in the flash capacitor remains intact. The extinguishing circuit consists of a capacitor, a choke, and an additional controllable circuit element. The capacitor is charged to a specific voltage at the beginning of the flash process. After attaining the predetermined value of the luminous time, the second circuit element is switched on. The capacitor discharges via the choke, the second circuit element, and the series thyristor. Therefore, the extinguishing current is in opposition to the flash current. As soon as the extinguishing current has risen to the value of the flash current, current through the series thyristor falls below the holding current; the series thyristor switches off, as desired, whereby the flash tube is also extinguished (See U.S. Pat. No. Re. 28,025).
As a controllable circuit element in order to switch on the extinguishing circuit, a thyristor, previously called an extinguishing thyristor, can also be utilized. However, it is also possible to use an extinguishing tube therefor. For this reason one can find gas-filled extinguishing tubes not only in conventional flash equipment which destroys the residual charge in the flash capacitor, but also in flash equipment retaining the residual charge in the flash capacitor.
Not only the flash tubes, but also the extinguishing tubes of the conventional flash equipment have an ignition electrode in order to ignite their gas-path. This ignition electrode, in the simplest case, consists of a metal ring mounted close to the cathode on the exterior of the wall of the tube. The secondary winding of the ignition transformer is connected with the cathode and the ignition electrode. This ignition transformer produces a high voltage impulse, such that the gas-filled tube is pre-ionized, and the discharge process between the main electrodes is thereby initiated. (See German Auslegeschrift No. 1,965,937, equivalent to U.S. Pat. No. 3,992,643).